6 



AMERICAN HYDKOIDS. 



By tracing downward some of these accessory tubes it is found that they end in hydrorhizal 

 elements. 



With these facts before us the entire manner of growth of such a fascicled stem becomes 

 clear. The planula attaches itself, forms the primary hydranth, and. grows upward in the form 



THE FASCICLED AND CANALICtU.ATED STEM CIK TI1E SKKTrt.AIUl >.K. 



Fig. 1. Fascicled stem of Srrtularellii gayi, partly dissected to show the origin of the peripheral tube (), and the origin of a. branch 



from the peripheral tube (f>). 

 Fig. 2. The same specimen with the axial tube (a () dissected out. a, a, peripheral tubes dissected nut springing from the buses of 



hydrotheca.-; a t, the axial tube; b, origin of branch; <, origin of branch from peripheral tube. 

 Fig. 3. Part of distal portion of colony, more highly magnified, showing origins of peripheral tubes at n, a. 

 Fig. 4. A single hydrotheca, greatly magnified, showing origin of peripheral tube at a. 

 Fig. 5. Part of stem of Selayinvpsis ornata, showing the extensive eanaliculation of tin' cu-nosarc; the dotted portions represent the 



(iriiMsareal canals. 

 Fig. 0. A branch of the same specimen, showing regular arrangement of the co^nosureal canals. 



of an ordinary mouosiphonic stem. It gives forth branches from the bases of the hydrothecse 

 which at first form ordinary branches. As the colony becomes larger some of the branches turn 

 immediately downward, become agglutinated to the original monosiphonic stem (now becoming 

 the axial tube), and finally form a bundle of tubes which entirely conceals the axial tube and its 



